Suspension and vibration isolation systems are commonplace and are used in a wide variety of applications in transportation and industry. Their purpose may be either to isolate the outside world from the vibrations of a payload, such as a motor or other vibrating device, or to isolate a payload from vibrations of its carrier. Often the isolation is desirable so as to avoid damage to equipment or discomfort to persons. Vibrations may also be a source of noise for sensors, and isolating a sensor from vibrations may significantly improve the quality of the sensed data.
In the context of this disclosure, a carrier is any object upon which a payload is to be mounted, carried or transported. A carrier may comprise a stationary or portable framework, or it may be a moving vehicle, a framework mounted to a vehicle, or a tethered object. When the carrier is a vehicle, it may traverse land, water or air. In the case of an airborne vehicle, the carrier may be a fixed or rotary wing aircraft, a lighter than air aircraft such as a blimp, zeppelin or aerostat, or a tethered airborne object. Tethered objects may include kites, or carriers towed or hung from an aircraft such as a bird, bomb or sonde as expressed in the parlance of airborne geophysics
In the context of this disclosure, a payload is any object which may be suspended from a carrier. The payload may be a source of vibration which is to be isolated, or it may be an instrument to be shielded from the vibrations of its carrier. The payload may comprise a framework, an instrument or instrument platform, or a separate suspension and vibration isolation system.
When used on a moving carrier vehicle, an objective of a vibration isolation system may be to apply minimal forces on the payload above a certain frequency while the payload tracks the general trajectory of its carrier below certain frequency. In the context of this disclosure, non-constant accelerations of the payload with respect to the carrier are understood to be vibrations.
In order that a vibration isolated payload may track its carrier, space must be provided within the carrier to permit the payload to move with respect to it. In a towed airborne carrier where space is limited, it is advantageous to have the suspension system out of the way of the payload motion so as to maximize available lateral motions of the payload while minimizing the dimensions of the carrier.
When a sensing or pointing instrument is used on a mobile carrier, the performance of the instrument may be affected by the motion of the platform. Data acquired with sensors such as gravity meters, gravity gradiometers, magnetometers, induction coils, radars, lidars, accelerometers, rotation rate actuators and various optical sensor or pointing devices such as telescopes, laser trackers and rangers, and cameras may be degraded by the presence of carrier vibrations. Vibration isolation of an instrument payload from the motions of its carrier may be of critical importance in the performance of the instrument. Such effects are very important in long range airborne tracking and pointing applications, gravity and gravity gradiometry, and in airborne electromagnetic measurements.
Vibration isolation in the context of airborne electromagnetic (AEM) surveys is an important consideration in the design of such survey equipment. The sensors which measure vector components of the magnetic field are extremely sensitive to angular jitter in the presence of the Earth's magnetic field. It is recognized in the present state of the art that effective isolation of the sensors from accelerations of their carrier can help to mitigate this jitter. Such jitter may be introduced by lateral and vertical motions of the carrier, and so may only be effectively suppressed through vibration isolation in three dimensions.
In practice, a suspension and vibration isolation system is securely mounted to a rigid framework of the carrier. The suspension and vibration isolation system provides the dual function of suspending the payload from this framework, while isolating the payload or carrier from vibrations in a certain frequency band. There are four essential quantities to be considered in any suspension and isolation system:
1.1. static load or weight-bearing capability,
1.2. softness or resonant frequency,
1.3. dynamic friction or loss, and
1.4. static friction or breakaway force.
The first, weight-bearing capability is characterized by the payload. The second, the softness or resonant frequency, is determined by the frequencies the payload is to be isolated from. Generally, effective isolation occurs at frequencies at least double the resonant frequency of the suspension and isolation system. The third, dynamic friction or loss, refers to the damping of the energy within the isolation system, and ideally energy will be damped without transmitting vibrations through the suspension to or from the payload. The fourth, static friction is a particular nuisance when damping low frequency vibrations, as the release of an object held by static friction causes a step acceleration on the payload. The effect of static friction can thus extend to frequencies above the resonant frequency of the suspension and isolation systems through the harmonic spectrum of the step. Static friction must be minimized as much as possible in AEM data acquisition as it prevents small amplitude vibrations at any frequency from being isolated from the payload.
In the established state of the art of AEM acquisition, vibration isolation only effectively eliminates jitter noise for magnetic measurements at frequencies above 20 Hz. Isolation methods in present state of the art AEM systems typically rely on elastometers, such as bungees, examples of which are provided in Canadian Patent No. 2,722,457 to Kuzmin and Morrison (“Double-suspension receiver Coil system and apparatus”) and U.S. Publication No. 2010/0237870 to Dodds (“Geophysical prospecting using electric and magnetic components of natural electromagnetic fields”). The invention of Turner et al (U.S. Pat. No. 6,369,573) relied on springs and damping fluid, and was never commercially viable. The invention of Barringer disclosed in U.S. Pat. No. 3,115,326 used gimbals to isolate a magnetic sensor coil from rotational motion. While Barringer's device may have been useful for acquiring AEM data in the 1960s, gimbal based devices have been largely abandoned for acquiring modern high-precision AEM data.
While many vibration isolation devices operate adequately in ranges well below 25 Hz, AEM measurements require vibration isolation solutions which minimize the electromagnetic noise caused by proximate electric currents and moving metal or magnetic ferrous metal parts. They must be robust to shock and thermal changes, be lightweight, compact and operate on low-power. In the case of high precision AEM measurements, where the sensors are located in a carrier towed on a cable tens of meters below an aircraft, a hanging suspension is highly advantageous as it minimizes the structural requirements (so reducing weight) and maximizes volume within the carrier where the payload may move. AEM survey equipment must function through rough landings and over a large thermal variation, often ranging from −20 C to 40 C. Because of these constraints the broad suite of vibration isolation technologies which have been developed for other applications are unsuitable for acquiring high precision AEM data.
Thus, while the general art of vibration isolation is well established, no suitable solutions have been found for acquiring high-precision AEM data in the sub-20 Hz range. The reason for this lies in the reliance on elastomers in the present AEM state of the art, and in the intrinsic properties of vibrational isolation systems. Vibrational isolation devices have a resonant frequency which lies substantially below the lower frequency of the vibrations to be isolated. As this resonant frequency is lowered, the range of motion that a vibrational isolation device requires will increase. This makes implementing elastomer-based low frequency vibration isolation difficult to do in practice, as long elastometers may be required. As a result, elastomeric based vibration isolation methods which work well in the acquisition of AEM data above 20 Hz do not work well for sub 20 Hz AEM data acquisition.
High-precision AEM measurements require a low-power, compact, non-magnetic, non-electric, robust and lightweight vibration isolation that can be suspended in a mobile carrier, criteria which make the use of elastomers appealing. While U.S. Pat. No. 6,196,514 “Large airborne stabilization vibration isolation system” to Kleinholtz uses air bearing pneumatic mounts, it is unsuitable to acquiring low-noise AEM data in a towed bird: It is too heavy and bulky to be installed in one; it requires on voice coil actuators (a source of electromagnetic noise), and it only provides friction-free vibration isolation in one direction, from the bottom.
Accordingly, it would be advantageous to have a light, compact vibration isolation system which could provide multi-directional vibration isolation to a payload suspended from above, and which could be installed and operated in a towed airborne carrier. It would be a further advantage in the acquisition of AEM data if the vibration isolation system could substantially isolate vibrations at frequencies above 3 Hz, if it could be operated with small amounts of electrical power, and if it could be substantially constructed out of resistive and non-magnetic components so as to minimize electromagnetic noise.